1. Field of the Invention
The invention relates to a fixing device and an image forming apparatus and, more particularly, to a fixing device having a fixing belt, and an image forming apparatus including the fixing device.
2. Description of Related Art
Conventionally, an image forming apparatus, such as a copier, a printer, a facsimile machine, and a combined machine including their multiple functions, is configured to form an image on a sheet by fixing a toner image transferred to the sheet by use of a fixing device. The fixing device performs a fixing process to heat the sheet while the sheet is passing through a fixing nip provided between a pair of fixing members placed in pressure contact with each other, thereby fixing the toner image on the sheet.
Some of the above fixing devices of the image forming apparatus are adapted to appropriately form an image even on an envelope which is apt to cause wrinkles or the like during the fixing process. For example, Japanese patent unexamined application publication No. 2008-3276 discloses an image forming apparatus configured, during image formation on an envelope, to adjust pressure contact force at a nip section in a fixing device and a feeding speed of the envelope to be conveyed to the nip section to form an image. Accordingly, the image can be formed even on an envelope with reduced wrinkles or other defects.
However, in the foregoing conventional art, there is a problem with difficulty in forming a good image on an envelope while suppressing wrinkles in the envelope. Specifically, the lower the pressure contact force at the nip section in the fixing device is set, the more the generation of wrinkles in the envelope can be suppressed when the envelope is passing through the nip section. However, the lower pressure contact force at the nip section in the fixing device is more likely to cause a deterioration in fixing property of a toner image to the envelope carrying the image.
The present invention has been made to solve the foregoing problems of the conventional techniques. Specifically, the present invention has objects of providing a fixing device and an image forming apparatus capable of appropriately performing a fixing process of a toner image on an envelope and preventing generation of wrinkles in the envelope carrying the toner image during the fixing process.